ISTANBUL — Turkish artillery fired shells into the northern Syrian region of Afrin on Friday as Turkey’s defense minister announced a military operation against American-backed Kurdish militias, despite warnings from the United States. The developments threatened to further destabilize a region fractured by seven years of war.

Fighters of the Turkish-backed Free Syrian Army, which were expected to spearhead the attack, were seen driving to the border around midday as artillery sounded on Turkey’s southern border with Syria. The Syrian fighters posted videos of men and vehicles preparing for the assault.

Turkey has said the Kurdish militias are part of a terrorist group that threatens its security, but it has struggled to win support for an intervention against them. Syria opposes Turkey’s intervention, and Syria’s deputy foreign minister, Faisal Mekdad, said the Syrian Air Force would destroy any Turkish airplanes used in an assault on Afrin.

Turkey’s defense minister, Nurettin Canikli, announced the Afrin operation in a live studio interview on the pro-government A Haber television network on Friday afternoon.

“I cannot comment here when the operation will start and from which direction,” he said. “What we are saying is this: This operation will be done. The terrorist organization will be cleansed from there.”

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A banner bearing the Turkish flag and an image of President Recep Tayyip Erdogan of Turkey during a demonstration in the rebel-held town of Azaz in northern Syria on Friday.CreditNazeer Al-Khatib/Agence France-Presse — Getty Images

Turkey has complained for months about United States support for the Syrian Kurds. Afrin is controlled by a Kurdish militia affiliated with the Kurdistan Workers’ Party, which has waged a three-decade insurgency against the Turkish state.

Turkey’s operation “has everything to do with domestic politics,” said Ahmet Han, associate professor in International Relations at Kadir Has University in Istanbul. The Kurdish threat was a genuine security concern, he said, but the government was also acting to maintain the support of a nationalist constituency at home.

United States officials have warned against the operation. The State Department spokeswoman Heather Nauert said on Thursday that the fight against remnants of the Islamic State, also known as ISIS, was the priority in Syria.

“We would call on — certainly on the Turks — to not take any actions of that sort,” Ms. Nauert said. “The focus needs to be on ISIS. So we don’t want them to engage in violence, but we want them to keep focused on ISIS.”

Separately, the United States is helping build a Kurdish-led border force in another part of Syria to protect land taken from the Islamic State. Turkey has also objected to that force, charging that it would place thousands of Kurdish militia fighters along Turkey’s border.

Russia, which supports the Syrian government, has been reluctant to back the Afrin operation. At a news conference at the United Nations on Friday, Sergey V. Lavrov, Russia’s foreign minister, denied reports that Russian observers were leaving Afrin in preparation for a Turkish offensive.

Without Russian support, Turkey’s intervention into Afrin would be fraught with risk, Turkish analysts said.

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A protest in Afrin against the Turkish operation.CreditYPG Press Office, via Associated Press

“There is a picture of diverging interests of Turkey and Russia,” said Kerim Has, a lecturer in Turkish-Russian relations at Moscow University. Russia did not want to get involved in a fight with the Kurds that is 100 years old, he said, and might not help Turkish troops if they got into trouble in Afrin. “Russia is not in Syria to solve the Kurdish issue.”

Turkey has made several incursions into Syria. In August 2016, Turkish forces battled the Islamic State for weeks to take control of the northern Syrian town of Al Bab, crucially with the help of Russian and American air support.

In recent days, Kurdish fighters and civilians have taken a defiant stance. On Thursday, there were demonstrations in Afrin against any Turkish offensive. The P.Y.D., the main political party of the Syrian Kurds, posted pictures of demonstrators waving the flag of the Syrian Kurdish militia, the Y.P.G.

In Manbij, an area that Turkey has threatened to take from a Kurdish-led militia known as the Syrian Democratic Forces, fighters said they were fortifying their positions.

Mohammed Billo, who lives in Afrin, said Turkish shelling had intensified. “If they start shelling the city center,” he said, “it will be a disaster.”

Another resident of Afrin, who requested his name not be published, said families had been leaving the city for the past three days, but Kurdish fighters at checkpoints had started to pressure civilians to stay. Among the mostly Kurdish residents of Afrin are thousands of Arabs displaced from villages near Afrin who want to leave the city and return to their hometowns or the city of Aleppo.

Pro-opposition activists warned that the Afrin operation was a mistake. Hassan Hassan, a Syrian analyst, wrote on Twitter: “This is the most irrelevant and stupid battle some of the opposition are getting involved in. Not only do the rebels have no dog in this fight, they’re hurting what’s left of their cause big time.”

Carlotta Gall reported from Istanbul, and Anne Barnard from Beirut, Lebanon. Hwaida Saad and Nada Homsi contributed reporting from Beirut, a New York Times employee from Damascus, Michael Schwirtz from the United Nations, and Karam Shoumali from Berlin.

A version of this article appears in print on , on Page A4 of the New York edition with the headline: Turkey Fires On Kurds In Syria, Ignoring U.S.. Order Reprints | Today’s Paper | Subscribe